1. Field of the Invention
The present invention relates to a canister for adsorbing fuel vapors.
2. Description of the Prior Art
FIG. 6 shows a conventional evaporation control system for limiting the discharge of fuel vapors into the atmosphere, in which the fuel vapors generated in a gasoline tank 1 mounted on a vehicle are collected in a fuel vapor adsorbing canister 4 (hereinafter referred to simply as a "canister") from an intake port 3 through a check valve 2 while the engine is stopped so as to be adsorbed and stored in a vapor adsorbing material, such as activated carbon, filling the canister 4, and in which a source of vacuum generated in an intake manifold 5 is utilized to remove the accumulated fuel vapors from the vapor adsorbing material through a purge port 6 and draw them into the intake manifold 5 for consumption in the engine combustion process while the engine is operating. Such an evaporation control system for limiting fuel vapor discharge into the atmosphere is disclosed in the specifications of, for example, Japanese Unexamined Utility Model Publication No. 123953/1982 and U.S. Pat. No. 3,884,204.
FIG. 7 shows a laterally disposed-type canister employed in the above-described conventional evaporation control system. The canister comprises a lateral cylindrical casing 7, a vapor adsorbing material layer 8 which is, for example, activated carbon, filling the casing; a filter 9 and a fixed, perforated retaining plate 10 which retains one side of the adsorbent layer 8; a filter 11 and a movable, perforated retaining plate 12 which are accommodated in the cylindrical casing 7 at another side opposite to the fixed retaining plate 10 in such a manner that they are slidable in the horizontal direction; and a spring 13 for pressing against the filter 11 and the movable retaining plate 12 to retain the adsorbent layer 8. The canister is also provided with an intake port 14 for fuel vapors, and an air port 15. The above-described type of canister has been disclosed in Japanese Unexamined Utility Model Publication No. 19188/1982.
In this lateral-type canister, if the adsorbent layer 8 is vibrated or the adsorbent layer crumbles, the voids in the adsorbent decreases, thereby decreasing the volume of the adsorbent layer. This creates a space 16 which extends between the two filters 9 and 11 and is formed between the entire upper surface of the descended vapor adsorbent layer 8 and the cylindrical casing 7. In consequence, part of the fuel vapors introduced into an intake chamber 17 from the intake port 14 passes the space 16 and is discharged into the atmosphere through an air chamber 18 from the air port 15 without being introduced in the adsorbent layer (8), with the result that the amount of fuel vapors discharged into the atmosphere increases to pollute the atmosphere. To overcome this problem, as has been described above, the above-described conventional canister is arranged so that the spring 13 constantly presses against the other side of the adsorbent layer 8 through the filter 11 and the movable retaining plate 12, compressing the adsorbent layer 8 so as to prevent the space 16 from occurring. However, the movable retaining plate 12, which is slidable horizontally in its upright state in the above-described manner, tends to become inclined due to vibrations. This inclination leads to an outflow of the adsorbent through a sliding portions defined between the slidable retaining plate 12 and the cylindrical casing 7, or leads to the jamming of the adsorbent therebetween even if a degree of the inclination is small, with the result that the smooth sliding thereof becomes impossible. If the canister is installed at the bottom of the automobile body, flying stones or the like hit the bottom of the canister, deforming the bottom wall thereof and impairing the smooth sliding of the movable retaining plate 12. If the movable retaining plate 12 cannot slide smoothly, the adsorbent layer 8 cannot be effectively pressed by the spring 13, leading to the occurrence of the space. With this type of canister, it is therefore necessary to restrict the position at which the canister is installed or to restrict the orientation in which the canister is installed in an automobile, so as to ensure that the movable retaining plate is not inclined.